Phagocytosis is critical for the removal of pathogens in host defense, and cells that are dying by apoptosis. Failure to clear pathogens results in infectious diseases. Failure to clear apoptotic cells (ACs) can result in developmental defects, autoimmunity or neurodegeneration. There are substantial gaps in our knowledge of the molecular mechanisms of phagocytosis. We do not know how phagocytes perceive and integrate AC-signals that promote phagocytosis. Until we fill this knowledge gap, developing new strategies to prevent or treat diseases that arise from defective phagocytosis will be difficult. Our long-term goal is to genetically dissect the molecular mechanisms of AC clearance. We use Drosophila as a model system, which has allowed us to advance our understanding of AC clearance by demonstrating novel roles for receptors on phagocytes, proteasomal degradation and calcium homeostasis in this process. Our rationale is that Drosophila can serve as a model to identify evolutionary conserved regulators of phagocytosis and shed new light on its molecular mechanisms, and tell us how AC-signals activate phagocytes. Our objectives are to identify novel positive and negative regulators of phagocytosis of ACs. Our central hypotheses are that: (1) apoptosis regulates the expression of Croquemort (CRQ), a CD36-related phagocytic scavenger receptor; (2) the substrates of Pallbearer, (PALL), an F-Box protein that promotes phagocytosis via its E3-Ubiquitin ligase activity, are negative regulators of phagocytosis; and (3) apoptosis regulates the subcellular localization of PALL, which acts upstream of CRQ. These are based on data showing that CRQ expression and PALL nuclear trafficking are apoptosis-dependent, that proteasomal degradation of PALL substrates promotes phagocytosis, and that CRQ is weakly expressed in pall mutants. Our specific aims are designed to (1) identify and study the role of CRQ regulators, (2) identify the PALL substrate(s) and study its (their) role in phagocytosis, and (3) study the trafficking of PALL in response to apoptosis and its relevance to AC clearance, using genetic and RNAi screens, biochemistry and molecular genetic techniques. This proposal wil advance our understanding of the molecular mechanisms of AC clearance, and identify regulators of this process that may serve as candidate targets for the development of new therapies designed to prevent or treat autoimmune, neurodegenerative and infectious diseases.